Cylinders, Pistons and Piston Rings Inspection
Visual inspection
Inspect cylinder walls for scratches, roughness or ridges which indicate excessive wear. If cylinder bore is very rough or deeply scratched, or ridged, rebore cylinder and use over size piston.
Cylinder bore diameter, taper and out-of-round
Using a cylinder gauge (1), measure cylinder bore in thrust and axial directions at two positions (“a” and “b”) as shown in the figure.
If any of the following conditions is noted, rebore cylinder.
1) Cylinder bore dia. exceeds limit.
2) Difference of measurements at two positions exceeds taper limit.
3) Difference between thrust and axial measurements exceeds out-of-round limit.
Limit on cylinder bore diameter
78.114 mm (3.073 in.)
Limit on cylinder taper and out-of-round
Limit: 0.10 mm (0.004 in.)
If any one of four cylinders has to be rebored, rebore all four to the same next oversize. This is necessary for the sake of uniformity and balance.
“a”: 50 mm (1.96 in.) “b”: 95 mm (3.74 in.)
Visual inspection
Inspect piston for faults, cracks or other damages.
Damaged or faulty piston should be replaced.
Piston diameter
As indicated in the figure, piston diameter should be measured at a position 19.5 mm (0.77 in.) (“a”) from piston skirt end in the direction perpendicular to piston pin.
Piston diameter specification
Standard size: 77.953 – 77.968 mm (3.0690 – 3.0696 in.)
Oversize (0.50 mm (0.0196 in.)): 78.453 – 78.468 mm (3.0887 – 3.0893 in.)
Piston clearance
Measure cylinder bore diameter and piston diameter to find their difference which is piston clearance. Piston clearance should be within specification as given follow. If it is out of specification, rebore cylinder and use oversize piston.
Cylinder bore diameters used here are measured in thrust direction at two positions.
Piston clearance
Standard: 0.032 – 0.061 mm (0.0013 – 0.0024 in.)
Limit: 0.161 mm (0.0063 in)
“a”: 19.5 mm (0.77 in.)
Ring groove clearance
Before checking, piston grooves must be clean, dry and free of carbon deposits.
Fit new piston ring (1) into piston groove, and measure clearance between ring and ring land by using thickness gauge (2). If clearance is out of specification, replace piston.
Ring groove clearance
   Standard Limit
Top ring 0.03 – 0.07 mm
(0.0012 – 0.0028 in.)
0.12 mm (0.0047 in.)
2nd ring 0.02 – 0.06 mm
(0.0008 – 0.0024 in.)
0.10 mm (0.0039 in.)
Oil ring 0.03 – 0.17 mm
(0.0012 – 0.0067 in.)
Piston Ring
Piston ring end gap
To measure end gap, insert piston ring (2) into cylinder bore and then measure the gap by using thickness gauge (1).
If measured gap is out of specification, replace ring.
Decarbonize and clean top of cylinder bore before inserting piston ring.
Piston ring end gap
Item Standard Limit
Top ring 0.20 – 0.35 mm
(0.0079 – 0.0138 in.)
0.7 mm (0.0276 in.)
2nd ring 0.30 – 0.45 mm
(0.0118 – 0.0177 in.)
1.0 mm (0.0394 in.)
Oil ring 0.20 – 0.70 mm
(0.0079 – 0.0276 in.)
1.5 mm (0.059 in.)
“a”: 120 mm (4.72 in.)